1. Field of the Invention
The present invention relates to an optical dispersion measurement apparatus and to a measurement method using the apparatus. More particularly, this invention relates to an apparatus for measuring the optical group velocity dispersion in single-mode optical fibers, and to a measurement method that uses the apparatus to measure the group velocity dispersion, the length of an optical fiber, and the distance to a measurement object.
2. Description of the Prior Art
Optical signals of different wavelengths propagating in an optical fiber travel at different velocities. This is referred to as group velocity dispersion, and increases the width of the optical pulses traveling along the optical fiber. Currently most optical communication applications use wavelengths in the 1.3-1.55 μm regions. In order to transmit signals over long distances, it is necessary to utilize optical fiber having optimal group velocity dispersion characteristics. With respect to methods of generating short pulses using optical fibers that compensate pulse chirp and utilize nonlinear and dispersion effects, the fibers used have to have optimal group velocity dispersion and length. Measurement of the group velocity dispersion in single-mode fibers is an essential part of achieving this.
The main methods of measuring optical-fiber group velocity dispersion are: a) the pulse-delay method; b) the interferometric method; c) the phase-shift method; and d) the baseband AM response method. Each of these methods are discussed below.
The pulse-delay method is described, for example, in “Pulse delay measurements in the zero material dispersion wavelength region for optical fibers,” by L. G. Cohen and Chinlon Lin (Applied Optics, Vol. 16, No. 12, pp. 3136-3139 (1977)). In this pulse-delay method, optical pulses having different center wavelengths are transmitted through the optical fiber being measured, and the relative pulse-delay time is measured to obtain the group velocity dispersion. The precision of this method is limited by the electrical bandwidth of the optical detector used to measure incident pulse width and relative delay time, and by the electrical bandwidth of the oscilloscope used for time-base measurements. It is therefore necessary to use detectors and oscilloscopes having high-frequency capabilities.
The interferometric method is described in “Interferometric method for chromatic dispersion measurement in a single-mode optical fiber,” by M. Tateda, N. Shibata and S. Seikai (IEEE Journal of Quantum Electronics, Vol. 17, No. 3, pp. 404-407 (1981)). In this method, optical pulses are divided into two components. One of these is used as a reference, and the other is transmitted through the fiber, passed through an optical delay circuit and reunited with the reference beam. By varying the reference beam delay, an interference fringe is obtained that is used to calculate the group velocity dispersion. Although this method does not require the use of high-frequency detectors or oscilloscopes, it is difficult to measure dispersion in long fibers, and is limited to measurements of fibers of up to 10 meters in length.
The phase-shift method of measuring group velocity dispersion is described, for example, in “Direct measurement of wavelength dispersion in optical fiber-difference method,” by K. Daikoku and A. Sugimura (Electronics Letters, Vol. 14, No. 5, pp. 149-151 (1978)). In this method, the beam from a single-mode laser is intensity-modulated and guided into the optical fiber to be measured. The incident light being subjected to the group velocity dispersion effect of the fiber, the phase of the baseband signal of the light exiting the fiber changes with the change in the optical wavelength. The method obtains the group velocity dispersion by using an oscilloscope to measure changes in the phase of the baseband signal against wavelength. The precision of measurements obtained by the method is limited by the modulation frequency, the bandwidths of the optical detector and the oscilloscope.
“Simple dispersion measurement technique with high resolution,” by B. Christensen, J. Mark, G. Jacobsen and E. Bodtker (Electronics Letters, Vol. 29, No. 1, pp. 132-134 (1993)) describes the baseband AM response method of measuring group velocity dispersion. A feature of the method is that it measures the group velocity directly. A beam from a single-mode laser is subjected to high-frequency amplitude modulation and input to the optical fiber. In the fiber, owing to the dispersion effect, the phase of two sidebands generated by the amplitude modulation undergoes change. As a result, at a given modulation frequency, the modulation undergoes a transition from amplitude to frequency modulation. The group velocity dispersion can be found by measuring this frequency. To use this method, the optical fiber concerned must be long, in the order of several tens of kilometers, and a network analyzer with a frequency capability of over ten gigahertz.
There is an optical gyro based on the Sagnac effect that is similar to the configuration of the present invention. Such a configuration is described in U.S. Pat. No. 5,056,919, for example. However, the objective of the apparatus configuration differs from that of the present invention. What the present invention has in common with the invention of the above disclosure is that both include means for inputting a light beam from a light-generating means to a first terminal of an optical distributor, means of outputting the input light from third and fourth terminals of the optical distributor, a substantially single optical path that links the third and fourth terminals, means of modulating light traveling from the third terminal to the fourth terminal and light traveling from the fourth terminal to the third terminal, means of outputting from a second terminal of the optical distributor light traveling from the third terminal to the fourth terminal and light traveling from the fourth terminal to the third terminal, and means of detecting intensity of light output from the second terminal.
However, another feature of that invention is to use a monochromatic optical source, whereas a feature of the present invention is to use a variable-wavelength optical source. The present invention also differs in that it includes a means to scan the light modulation frequency and a means to correlate the optical intensity of the light output from the second terminal as a periodic function of the modulation frequency. The present invention's inclusion of means for varying the modulation frequency makes it possible to also measure the effect of the dispersion of fibers comprising the interferometer by thus varying the frequency. Moreover, in contrast to an optical gyro having a non-replaceable measurement object and which is utilized for measuring rotation and the like, in the present invention the measurement object can be replaced and forms part of the interferometer. This is a major difference with respect to measurement of optical characteristics. Furthermore, while in the case of an optical gyro modulation is performed at a fixed eigen frequency (with f being a characteristic eigen frequency, f=c/(2nL), where n denotes refractive index, L denotes loop length and c denotes velocity of light in a vacuum), the measurement method of the present invention uses scanning at a frequency in the radio frequency range that is much higher than an eigen frequency to measure the optical characteristics of the object fiber.
In the prior art optical dispersion measurement apparatus and methods, in the measurement of group velocity dispersion using the pulse-delay method, phase-shift method or baseband AM response method, it was necessary to use an optical detector, oscilloscope or network analyzer with a broad range of frequency capabilities of from several to several tens of gigahertz.
Based on a consideration of the foregoing, an objective of the present invention is to provide an optical dispersion measurement apparatus, and a method using the same, that has a simple configuration that helps to reduce the cost of measuring optical dispersion.